The present invention relates to an improved method for the recovery of oil from subterranean, viscous oil-containing formations. More particularly, the invention relates to a thermal oil recovery process utilizing mixtures of steam and oxygen.
Many oil reservoirs such as heavy oil or tar sand formations have been discovered which contain vast quantities of oil, but little or no oil has been recovered from many of them because the oil present in the resrvoir is so viscous that it is essentially immobile at reservoir conditions, and little or no petroleum flow will occur in a well drilled into the formation even if a natural or artificially induced pressure differential exists between the formation and the well. Some form of supplemental oil recovery must be applied to these formations which decrease the viscosity of the oil sufficiently that it will flow or can be dispersed through the formation to the production well and therethrough to the surface of the earth. Thermal recovery techniques which are quite suitable for viscous oil formations include steam flooding and in-situ combustion.
Steam may be utilized for thermal stimulation for viscous oil production by means of a steam drive or steam throughput process, in which steam is injected into the formation on a more or less continuous basis by means of an injection well and oil is recovered from the formation from a spaced-apart production well.
Conventional in-situ combustion involves drilling of at least two substantially vertical wells into the formation, the wells being separated by a horizontal distance within the formation. One of the wells is designated an injection well, and the other a production well. The recovery of oil is accomplished by raising the temperature of the in-place oil adjacent the injection well to ignition temperature by some suitable means, e.g., with some type of a conventional downhole heater/burner apparatus, or by steam injection and then supporting combustion by injecting an oxygen-containing gas such as air, oxygen-enriched air, oxygen mixed with an inert gas, or substantially pure oxygen. Thereafter, the injection of the oxygen-containing gas is continued so as to maintain the high temperature combustion front which is formed, and to drive the front through the formation toward the production well. As the combustion front moves through the formation, it displaces ahead of it the in-place oil with reduced viscosity as well as other formation fluids such as water and also combustion gas produced during the combustion process. These fluids are recovered from the formation via the production well.
As an improvement to the in-situ combustion process, water or steam may be injected with the oxygen-containing gas which is referred to as wet combustion. Wet combustion reduces the temperature of the in-situ combustion reaction resulting in less consumption of the crude oil present in the formation and increasing the sweep efficiency of the combustion front. U.S. Pat. Nos. 3,976,137, 4,114,690, and 4,127,122 disclose injection of a mixture of an oxygen-containing gas and steam for low temperature, controlled oxidation viscous oil recovery.
The low temperature oxidation (LTO) process has the disadvantage of "wasting" oxygen, i.e., it is a partial oxidation that adds oxygen to hydrocarbons to form oxygenated functional groups such as --OH, --COOH, &lt;C.dbd.O, --CHO, etc., instead of causing oxidation all the way to carbon dioxide and water which releases the most heat of combustion. Low temperature oxidation is also associated with the formation of viscous products, usually several orders of magnitude more viscous than the original oil which can lead to plugging of the formation that decreases injectivity of the fluid mixture.
The present invention provides an improved thermal recovery method whereby viscous oils can be recovered more efficiently by injecting a mixture of an oxygen-containing gas and steam wherein the ratio of molecular oxygen in the oxygen-containing gas to steam is about 5 volume percent to 25 volume percent of the injected fluids.